FIG. 1A illustrates a cross-sectional view of a conventional non-volatile charge trap memory device where an oxide-nitride-oxide (ONO) stack is used to store charge in a nitride layer having a high density of charge trap states, forming a semiconductor-oxide-nitride-oxide-semiconductor (SONOS) transistor. In functional terms, the first “Semiconductor” refers to the channel region of the substrate, the first “Oxide” refers to the tunneling layer, “Nitride” refers to the charge trapping layer, the second “Oxide” refers to the blocking layer and the second “Semiconductor” refers to the gate layer. The charge stored in the nitride trapping layer enables a SONOS transistor to provide non-volatility memory (NVM).
As further shown in FIG. 1A, non-volatile charge trap memory device 100 includes a SONOS gate stack 104 including a conventional ONO portion 106 formed over a silicon substrate 102. Non-volatile charge trap memory device 100 further includes source and drain regions 110 on either side of SONOS gate stack 104 to define a channel region 112. SONOS gate stack 104 includes a poly-silicon gate layer 108 formed above and in contact with ONO portion 106. Poly-silicon gate layer 108 is electrically isolated from silicon substrate 102 by ONO portion 106. ONO portion 106 typically includes an oxide tunneling layer 106A, a nitride or oxynitride charge trapping layer 106B, and an oxide blocking layer 106C overlying charge trapping layer 106B.
One limitation of conventional SONOS transistors is quality of the dielectric employed for the charge trapping layer 106B. While a poor quality dielectric provides a beneficially high density of trap states for charge storage, the poor quality dielectric is detrimentally leaky and unable to retain the trapped charge. This leakage limits the retention time of the charge trap memory device. It is desirable to selectively tailor the quality of the charge trapping layer 106B in a manner that provides a density of trap states sufficient for charge storage and a reduced rate of trapped charge leakage.
A conventional method for forming the ONO portion 106 of FIG. 1A is shown in FIG. 1B. In operation 151, the tunneling layer is grown by a thermal oxidation from the silicon substrate. A high quality silicon dioxide layer may be produced with such a process. Then, at operation 152, to form an oxynitride charge trapping layer, a low pressure chemical vapor deposition (LPCVD) process is employed while a similar deposition process is typically used at operation 153 to form a blocking layer. In an LPCVD deposition, the quality of the deposited layer is typically limited by the non-equilibrium deposition mode. Thus, even when deposition conditions are tuned in an effort to deposit a good quality oxynitride layer, there is still a significant quantity of hydrogen and non-terminated bonds within the layer.